1. Field of the Invention
The present invention generally relates to optical scanners and imaging apparatuses, and more particularly to an optical scanner that forms a light spot on a scanning surface by using optical deflection and scanning means and a scanning and imaging optical system and optically scans the scanning surface with the light spot, and an imaging apparatus using such an optical scanner.
2. Description of the Related Art
Optical scanners that deflect a light beam emitted from a light source modulated based on image data in a main (primary) scanning direction using optical deflection and scanning means such as a rotary polygonal mirror, focus the deflected light beam into a light spot on a scanning surface (surface to be scanned) using a scanning and imaging optical system such as an fθ lens, and scan the scanning surface with the light spot formed thereon are widely known in relation to imaging apparatuses such as optical printers, optical plotters, and digital copiers.
In an imaging apparatus using such an optical scanner, generally, an imaging process including a step of writing an image by optical scanning is performed. The quality of the formed image depends on the quality of the optical scanning. The optical scanning quality depends on the scanning characteristics of the optical scanner in the main and sub scanning directions.
The scanning characteristics in the main scanning direction include the constant velocity characteristic of optical scanning.
In the case of using a rotary polygonal mirror as optical deflection and scanning means, for instance, a light beam is deflected at a constant angular velocity. Therefore, a scanning and imaging optical system having the fθ characteristic is employed in order to realize the constant velocity characteristic of optical scanning. It is not easy, however, to realize a perfect constant velocity characteristic of optical scanning due to other characteristics required of the scanning and imaging optical system. Therefore, in actual optical scanning, optical scanning is not performed at a perfect constant velocity, so that the constant velocity characteristic as a scanning characteristic is accompanied by “deviation or difference from ideal constant velocity scanning.”
The scanning characteristics in the main scanning direction are generated not only by the above-described insufficient correction of the fθ characteristic, but also by: a decrease in the accuracy of optical components or the accuracy of attachment to the housing of the optical scanning system; changes in the optical component properties caused by environmental changes such as changes in temperature and humidity inside the optical scanner; and the shift of a photosensitive body or an intermediate transfer body, which substantially forms the scanning surface, in the direction of its rotation axis.
The scanning characteristics in the sub scanning direction include “scanning line curving” and “scanning line tilting.” A scanning line is the trace of movement of a light spot on a scanning surface, and ideally, is a straight line. The optical scanner is designed so as to have a straight scanning line. As it is, however, it is common that the scanning line is curved due to processing or assembly error. This curving of the scanning line is referred to as “scanning line curving,” and the degree of curving is referred to as the “scanning line curving characteristic.”
In the case of using an “imaging mirror” as a scanning and imaging optical system so as to form an angle in the sub scanning direction between the direction in which the deflected light beam is made incident on the imaging mirror and the direction in which the deflected light beam is reflected therefrom, scanning line curving occurs in principle. In the case of forming the scanning and imaging optical system as a lens system, scanning line curving is unavoidable in multi-beam scanning where the scanning surface is optically scanned by a plurality of light spots separated in the sub scanning direction.
The “scanning line tilting,” which is a phenomenon where the scanning line does not cross the sub scanning direction correctly at right angles, is a type of scanning line curving. Accordingly, in the following description, “scanning line tilting” is included in “scanning line curving” unless otherwise indicated.
Imperfection of the constant velocity characteristic of optical scanning distorts a formed image in the main scanning direction, and scanning line curving distorts a formed image in the sub scanning direction. When a single optical scanner writes and forms a monochrome image, the formed image includes no “visually recognizable distortion” if scanning line curving and imperfection of the constant velocity characteristic (or the deviation or difference from the ideal constant velocity scanning) are suppressed to a sufficient extent. However, the image is better with less distortion even if the distortion is visually unrecognizable.
Even in a monochrome image, however, scanning line curving becomes an unignorable problem if image linearity is considered important as in the case of a CAD (computer-aided design) output because even a slight curve of the scanning line significantly degrades the quality of the output image.
Conventionally, in color copiers, images of three colors of magenta, cyan, and yellow or four colors of those three colors plus black are formed as color component images, and a composite color image is formed by superimposing those color component images.
In forming such a color image, a so-called “tandem-type” imaging method is employable. In this imaging method, color component images are formed on different photosensitive bodies by different optical scanners (or different optical scanner parts), respectively. According to this imaging method, however, if the degree of scanning line curving or tilting differs between the optical scanners, an abnormality called “color misregistration” where colors are superimposed inappropriately on top of each other is caused to appear in the formed color image by a slight misalignment of the scanning lines of the optical scanners, thereby degrading the quality of the color image, even if the scanning line curving or tilting is corrected to an acceptable level in each optical scanner.
Further, the “color misregistration” may appear as a phenomenon where a desired tone cannot be obtained in the color image.
In recent years, a special surface, typically, an aspheric surface, has been commonly employed in the imaging optical system of the optical scanners in a bid to improve the scanning characteristics. Accordingly, a low-cost imaging optical system of a resin material, with which such a special surface can be formed easily, is widely used.
When the resin imaging optical system is affected by changes in temperature or humidity, the shape and the refractive index of its element are subject to change. Therefore, the optical properties of the imaging optical system are subject to change. These changes in the optical properties also change the scanning line curving characteristic and the constant velocity characteristic. Then, in the case of successively forming tens of color images, for instance, the imaging apparatus operates continuously to increase its internal temperature, so that the optical properties of the imaging optical system change to gradually change the scanning line curving characteristic and the constant velocity characteristic of each optical scanner. This may cause the phenomenon of color misregistration, thus resulting in a great tone difference between a color image obtained at the beginning and a color image obtained at the end.
Japanese Laid-Open Patent Application No. 2001-215437 discloses a technology of forming an exposure distribution in the sub scanning direction on a scanning surface or in a scanning region. Japanese Laid-Open Patent Application No. 2000-235290 discloses a technology of adjusting a scanning line position in the sub scanning direction by controlling the modulation timing of a light source.
Further, Japanese Laid-Open Patent Application No. 8-313941 discloses a technology relating to a liquid crystal deflection element.